The invention relates to the field of Surface Plasmon Resonance and waveguide phenomena. In particular the invention is directed to beam shifting surface plasmon resonance or plasmon-waveguide resonance systems and methods.
It is understood that when a light-reflecting surface is coated with a thin metallic coating, then light at a specific incident angle can excite the electrons in the metal. This results in localized fluctuations of the electron density known as surface plasmons. The light energy transferred to the metal coating during excitation results in an attenuation of the reflected light intensity. The incident angle and degree of the attenuation depends on the wavelength of the exciting light and the thickness and optical properties of the interface in contact with the metal coating.
The important optical properties of such an interface include the absorbance at the excitation wavelength (extinction coefficient), the refractive index, and the thickness of the interface. The effective distance of Surface Plasmon penetration is only several hundred nanometers (nm) so only the environment at the surface is detected. This property makes Surface Plasmon Resonance (SPR) and Plasmon Waveguide Resonance (PWR) ideal for measuring surface and interfacial chemistry, as well as the properties of thin film coating properties (including molecular films).
It is also understood that gold and silver are two metals that produce strong SPR signals. Under similar conditions the SPR electric field in the sample produced by silver is more than 2 times stronger than gold resulting in much sharper resonances and greater sensitivity. However, the chemical reactivity of silver renders it inappropriate for many applications. Therefore many applications utilize gold as the metallic coating.
PWR is essentially a species of SPR however, PWR techniques utilize one or more dielectric coatings (e.g., silica dioxide) over the metallic coating. The appropriate dielectric coatings serve as both a shield and an xe2x80x9coptical amplifierxe2x80x9d. PWR allows the use of silver as the metallic coating or layer, with its improved optical properties but without suffering from its undesirable chemical properties.
SPR systems utilize specific light polarizations (e.g., p-polarization) in reference to the sample plane to produce resonance signals. In PWR systems, the appropriate dielectric coating also serves as an optical amplifier resulting in additional sharpening of the resonance spectrum, and more importantly, allowing light polarizations both parallel (s-polarization) and perpendicular (p-polarization) to the sample plane to produce resonance signals. For example, a silver layer 50 nm thick produces an SPR spectrum that is roughly 2 degrees wide.
The same layer when properly coated produces two different PWR spectra with the two light polarization, that are more than an order of magnitude sharper. The unique characteristics of PWR allow more information about the sample properties to be obtained at much higher sensitivities. In particular, probing optically anisotropic samples requires the capabilities that PWR offers. Thus, for anisotropic samples, the refractive index and extinction coefficient have different values for polarizations parallel and perpendicular to the sample plane, yielding information about molecular orientation within the sample.
See e.g., U.S. Pat. No. 5,521,702xe2x80x94Salamon, et al.xe2x80x94which discloses the use and formation of a biocompatible film composed of a self-assembled bilayer membrane deposited on a planar surface in connection with SPR techniques. See also, U.S. Pat. No. 5,991,488xe2x80x94Salamon, et al. which discloses a prism having a metallic film coated with a dielectric layer used to provide a surface plasmon wave.
Most SPR instruments do not record the SPR spectra but reduce the information to only the relative angle at which the resonance peak is detected. This approach eliminates the possibility of determining optical properties. Changes in the relative angle are assumed to correlate to changes in the refractive index of the sample layer (measured with only one polarization) due to mass moving into and out of the layer. However, relative angle measurements assume that the sample is isotropic and that the thickness and absorbance (or scattering) are constant. Unfortunately these assumptions are usually not correct in practical applications and can result in misleading data and erroneous conclusions. In addition, the molecular interactions resulting in changes in mass of the sample also usually influence the molecular organization. As an example, conformation changes occurring without net binding will result in changes in the relative angle. Further, changes in the bulk solvent will produce changes in the relative angle and can appear as binding effects. One way to avoid such misleading measurements is to use both polarizations and to analyze the full resonance spectrum.
There are a number of applications for a PWR spectrometer. For example, PWR devices can be used to probe molecular interactions (i.e. binding followed by structural alterations induced by binding) within anisotropic interfaces and thin films, including: optical coatings, lipid bilayers, proteins and peptides inserted into lipid bilayers, and others. It can also be used the way as a conventional SPR instrument to follow changes in the angular resonance peak position.
The invention is directed to improvements in PWR technology thereby yielding improved results such as more accurate measurements and increased automation of PWR systems and methods.
The invention is directed to a surface plasmon resonance system and method. The system has at least one light source operable to generate a source beam, a prism having a rear surface at least partially coated with a metallic coating, at least one layer under test, the layer under test being positioned on the rear surface of the prism, wherein the source beam is directed at the layer under test thereby defining an angle between the source beam and the rear surface of the prism. The system also has a detector operable to detect light that is reflected and/or scattered by the layer under test, wherein the source beam is directed towards the layer under test at a selected angle, and the source beam is shifted along a first plane (e.g., horizontal or vertical) to illuminate at least a portion of the layer under test based on the selected angle and the position of the layer under test on the rear surface of the prism.
In one embodiment the system has a turntable operable to rotate about an axis of rotation and adjust the angle between the source beam and the rear surface of the prism to the selected angle, wherein the prism is coupled to the turntable. In this embodiment, the system also has a mirror coupled to a linear slide (e.g., microslide), wherein the linear slide is operable to linearly displace the mirror, the mirror being operable to direct the source beam towards the prism and the linear slide being operable to shift the source beam and illuminate at least a portion of the layer under test based on the selected angle and the position of the layer under test on the rear surface of the prism.
In another embodiment the system has a steering mirror coupled to a linear slide, wherein the steering mirror is operable to rotate about an axis of rotation, wherein the linear slide is operable to linearly displace the steering mirror, the steering mirror being operable to direct the source beam towards the prism and adjust the angle between the source beam and the rear surface of the prism to the selected angle, the linear slide being operable to shift the source beam and illuminate at least a portion of the layer under test based on the selected angle and the position of the layer under test on the rear surface of the prism.
In yet another embodiment, the system has a first and second steering mirror, wherein each steering mirror is operable to rotate about an axis of rotation, wherein the first and second steering mirror are operable to direct the source beam towards the prism, adjust the angle between the source beam and the rear surface of the prism to the selected angle and shift the source beam and illuminate at least a portion of the layer under test based on the selected angle and the position of the layer under test on the rear surface of the prism.
Another aspect of the invention provides for a metallic coating on the rear surface of the prism that is at least partially coated a dielectric layer. The system can also utilize a prism that has a rear surface that is partially coated with a metallic coating, thereby defining a coated portion and an uncoated portion.
Another aspect of the invention provides for at least a first and second layer under test are formed on the rear surface of the prism. For example, a first layer under test can be formed on the coated portion and second layer under test can be formed on the uncoated portion. In the alternative, a sample layer can be formed on the coated portion and a coated reference layer can be formed on the coated portion. In yet another alternative, a sample layer can be formed on the coated portion and a bare reference layer is formed on the uncoated portion. In yet another alternative, a sample layer can be formed on the coated portion, a coated reference layer can be formed on the coated portion, and a bare reference layer can be formed on the uncoated portion.
Another preferred aspect of the invention provides for source beam shifting along a first plane (e.g., horizontal) and a second plane (e.g., vertical) so that the rear surface of the prism can be illuminated in two-dimensional array like fashion. To this end, first and second mirrors can be provided to shift the source beam along the second plane.